1. Field of the Invention
The present disclosure relates to a power amplifier and, more particularly, to a power amplifier that is suitable for use as a guitar amplifier, and an input signal adjusting method.
2. Description of the Related Art
In recent years, power amplifiers using semiconductor devices such as class-D amplifiers have come into wide use as audio power amplifiers. For example, in class-D amplifiers, an output-stage transistor is on/off-switched by a PWM pulse sequence that is generated through pulse width modulation according to an input audio signal and a current is supplied to a speaker from a power source via the output-stage transistor. Class-D amplifiers can supply power to speakers efficiently because they are very low in output impedance and are hence very low in the loss of the output-stage transistor. More specifically, whereas the internal impedance of speakers is 8 to 16Ω, the output impedance of class-D amplifiers is approximately equal to 0Ω.
Furthermore, in class-D amplifiers, usually a voltage that is input to a speaker is fed back to an input portion and the gain of the class-D amplifier is controlled according to the difference between the input audio signal and the negative feedback signal. Therefore, class-D amplifiers can perform constant-voltage driving in which a voltage corresponding to an input audio signal is supplied to a speaker independently of the frequency. An exemplary document among related art documents relating to class-D amplifiers is WO2003/090343.
Even at the present time when power amplifiers using semiconductor devices are in common use, vacuum tube amplifiers are still used frequently as guitar amplifiers that amplify an output signal of an electric guitar. This is because vacuum tube amplifiers allow speakers to emit, at high volume levels, sound with quality preferred by players, which is in contrast to power amplifiers using semiconductor devices.
A factor that has great influence on the quality of reproduction sound in the case where vacuum tube amplifiers are used is characteristics of power sources that are use for vacuum tube amplifiers. More specifically, for vacuum tube amplifiers, power amplifiers having an internal capacitor and a relatively large internal impedance are used. As a result, vacuum tube amplifiers can provide a large instantaneous maximum power relative to a continuous-wave maximum power and make it possible to emit sound at high volume levels instantaneously. Thus, vacuum tube amplifiers realize reproduction sound having unique quality by virtue of their constant-current output characteristic and the above characteristics of a power source.
However, vacuum tube amplifiers are inconvenient to handle because they are large in size and heavy. Vacuum tube amplifiers employ, in an output stage, a pentode that is high in output impedance and hence suffer a large loss in the output stage, which raises another problem that they cannot drive loads (speakers) efficiently.
Therefore, to drive loads efficiently, it is necessary to use class-D amplifiers. In this case, to allow class-D amplifiers to perform sound reproduction of the same level as vacuum tube amplifiers do, a method is conceivable that employs, for class-D amplifiers, the same power sources as used for vacuum tube amplifiers. However, power sources for vacuum tube amplifiers have large internal impedance values than ones for class-D amplifiers. Where such a power source having a high internal impedance is used for a class-D amplifier, a problem arises that it is difficult for the class-D amplifier to perform constant-voltage driving and hence it cannot exercise its function properly.